Transmission lines often suffer from crosstalk which results from the parasitic coupling between nearby conductors. The coupling between the conductors can be either capacitive or inductive in nature and is typically associated with integrated circuit packages, connectors, and printed circuit traces positioned in parallel with one another.
A particular type of data transmission system transmits signals differentially along pairs of conductors. A differential driver receives data at an input and generates a corresponding differential signal on a pair of outputs (i.e., an inverting output and a non-inverting output). The differential signal is transmitted across a pair of conductors and received by a differential receiver, which rejects signals that are present on both conductors (referred to as common mode signals). This differential transmission system helps reduce noise that is induced on both conductors, however coupling noise still exists on the conductors. For example, a differential pair of conductors is likely to have noise when passing through a connector. As transmission rates increase, the resulting increase in signal edge rates will aggravate this problem.
Existing techniques to reduce coupling between conductors include adding ground conductors between signal conductors or positioning the signal conductors farther away from one another. However, the addition of ground conductors between signal conductors increases the number of conductors, thereby increasing the cost and complexity of the system. Further, if the conductors are traces on a printed circuit board, the addition of ground conductors between signal conductors increases the printed circuit board area required to route all of the conductors. Positioning the signal conductors farther away from one another increases the size of the printed circuit board, connector, integrated circuit package, or other device that handles the conductors.
FIG. 1 illustrates a prior art system in which inductive coupling occurs between various conductors. The system uses differential signaling to transmit three bits of data across three differential line pairs. The inductors shown coupled in series with the various conductors represent the inductances inherent in the conductors. The three data bits are designated as a, −a, b, −b, c, and −c. An integrated circuit 100 receives or generates the three data bits that are provided to differential drivers 108, 110, and 112. The signals generated by the differential drivers 108-112 are transmitted through a connector 102, another connector 104, to another integrated circuit 106. Each of the three pairs of conductors is coupled to one of three differential receivers 114, 116, or 118, which are contained in integrated circuit 106. For example, the system shown in FIG. 1 may be a backplane having the connectors 102 and 104 mounted to the backplane. In this example, integrated circuits 100 and 106 are mounted on a substrate (such as a printed circuit board) which is electrically coupled to one of the connectors 102, 104.
In the example of FIG. 1, coupling noise is caused by inductive coupling. This coupling corresponds to parasitic mutual inductance which occurs due to the physical construction of the integrated circuit packages and connectors in the system. In FIG. 1, the mutual inductance is represented by inductive coupling coefficient K1, K2, and K3. Since the two connectors 102 and 104 are the same, they have the same inductive coupling coefficient (K2). The two integrated circuit packages 100 and 106 are different, so they each have a different inductive coupling coefficient (K1 and K3).
The signal quality in the system of FIG. 1 is degraded due to the parasitic mutual inductance. For example, if signals a, −a and c, −c switch while signals b, −b are quiet, a noise signal will appear at the receiver end of the line carrying b, −b. This type of noise signal reduces the timing and voltage margins of data arriving at the differential receiver 114, 116 or 118.
The system and method described herein addresses these and other problems by reducing the coupling noise on conductors positioned near one another.